OFDM has become widely accepted as a modulation mechanism and was adopted for the IEEE 802.11a modulation standard. However, there are a number of known drawbacks to OFDM. One disadvantage is that conventional OFDM inherently exhibits a large peak to average power ratio (PAPR). The PAPR of an OFDM system is given by 10 log(N) where N is the number of subcarriers. Thus, for a 64 subcarrier system, the PAPR≈18 dB. Due to this large variation from peak to average that must be accommodated, an OFDM transmitter generally requires a linear amplifier. Since linear amplifiers are less efficient in transmission of signals with a large PAPR, power is often wasted. This can be a significant concern in battery-powered equipment.
A second known drawback with OFDM systems is that as the number of subcarriers increases, for a given bandwidth, the requirements on oscillator phase noise increases. Such oscillators generally have requirements for a low variation in oscillator frequency (over all operational conditions) and a low level of phase noise. These restrictions can place lower limits on the cost of the oscillator circuitry.
A third known drawback with OFDM systems is that intercarrier interference (ICI) is easily introduced as a result of Doppler effects caused by movement of the transmitter, receiver or objects in the surrounding environment.